It is known in the art that when water is present in regions of the insulation structure of an electrical cable, water/chemical trees develop and propagate, causing deterioration of the cable insulation. Regions of localized high electrical stress caused by voids, contaminants and protrusions from the conductor and insulation stress control layers are particularly susceptible. Water present in the spaces between the strands of a multi-stranded conductor significantly accelerates the propagation of water/chemical trees in the insulation. It is therefore desirable to fill all spaces between such wires with a filling compound to minimize or prevent ingress and movement of water in such spaces. See, for example, U.S. Pat. Nos. 4,095,039; 4,104,480; 4,145,567; 3,943,271.
U.S. Pat. No. 5,049,593, assigned to the assignee of the present invention and incorporated by reference herein, describes an improved polymeric filling compound for use in electrical cables. Water swellable particles are admixed with or applied to the surface of the polymeric compound providing a more effective block against the ingress and movement of water. In one example, the filling compound and water swellable powder is applied over a first layer of wires during stranding of the wires. A second layer of wires is stranded over the first layer of wires and the filling compound. The second layer is similarly coated and an outer layer of wires is then stranded over the first two layers of wires and filling compound. A rotating wire cage carrying bobbins of strand wires is used to strand the wires over the previous layer of wires and filling compound.
Tubular stranders, wherein the bobbins of strand wire are arranged linearly within a rotating frame, may also be used to strand wires. They may be preferred because of their speed. To fill the interstices between the wires of a cable stranded by a tubular strander, filling compounds are typically applied to the core wire or wires upstream of the tubular strander. The coated core is drawn through the tubular strander to the closing die guided by rollers on the tubular strander.
As the coated core is drawn through the tubular strander, the filling compound is prone to contamination. It may also lose uniformity, drip from the core wire or be removed from the core wire by contact with portions of the tubular strander. Filling compounds have also been applied to cable cores upstream of a rotating wire cage strander. See, for example, U.S. Pat. No. 3,923,003. Such a system is prone to the same problems described above with respect to the tubular strander.
U.S. Pat. No. 4,406,114 attempts to address the problems associated with the application of filling compound, such as a corrosion inhibitor, to a core wire upstream of a strander, by providing an applicator within the tubular strander, near its downstream end where the core wire is wound with strand wire. The core wire is drawn through the applicator, coated, and immediately wound with one or more strand wires, which is said to avoid dripping, smearing, contamination and premature deformation of the corrosion inhibitor. Since the applicator is within the strander, however, storage tanks for the corrosion inhibitor must be provided within the strander, as well. To refill such tanks, the operation of the strander must be stopped. The stranding operation cannot, therefore, be run continuously.
U.S. Pat. No. 3,085,388 shows a rotating applicator for applying filler compound to a core wire of a stranded conductor, which is located downstream of a tubular strander. Separate passages through the applicator are provided for the strand wires to pass through, uncoated. The strand wires are stranded around the core wire in a die after passing through the applicator. While alleviating certain of the problems associated with applying compound to a core, the patent does not show how to apply a layer of water absorbing particles over the filler compound.
A method and apparatus for coating a core wire or wires with water blocking compound and water absorbing particles, which may be positioned downstream of a strander, would be advantageous.